The serene, spacious spaceship Discovery in Stanley Kubrick's 2001: A Space Odyssey is pure space fantasy. The real deal is a crucible (even without a homicidal supercomputer). Astronauts travel light, but they bring their psychological baggage with them.

Tight quarters, lack of privacy, and disruption of the terrestrial cycle of night and day can upset even the hardiest astronaut. And we've seen the behavior that shows it. Russian Valentin Lebedev says he spent 30 percent of his seven-month 1982 stint on Mir mediating intracrew conflicts.

If this is what happens to astronauts in orbit for months, what can NASA expect on even longer missions? A Mars voyage like the one President Bush proposed in January would keep six or seven men and women cooped up for two and a half years - far longer than Mir's 438-day record. The stress of such isolation could leave the crew vulnerable to life-threatening human errors.

In response, NASA has drawn up the inch-thick Bioastronautics Critical Path Road Map. The latest edition, published in April, identifies 50 behavioral health and performance risks arising from long missions. Here are some of the big ones.

Sleep deprivationA spaceship's noise and bright lights can play havoc with sleep. Moreover, in the absence of regular periods of sunlight and darkness, astronauts tend to sleep an average of six hours in every 24. "Acute or chronic degradation of sleep quality or quantity," the Road Map notes, can bring on dangerous behavior.

Catfights It's not uncommon for crew members to "hate each other's guts," says Larry Palinkas, a NASA consultant and expert on the psychological effects of isolation. The lack of "social novelty," he says, can magnify insignificant quirks, like the way a person chews food or tells the same story over and over.

Bad newsA grief-stricken crew member could imperil everyone on board. Thus, Mars mission planners face a dilemma: Break news of a death in the family by radio and possibly endanger the vessel and crew, or wait - and risk alienating the astronaut when the deception is later revealed. Cosmonaut Vasily Tsibliyev lost his sister while he was in orbit in 1993, and his stepfather while on Mir in 1997. In both cases, ground control did not tell him until his return.

Psychotic breaksLoneliness, injury, and failure to complete a mission can trigger melancholy or worse. If an astronaut becomes violent or suicidal, protocol calls for restraints and sedatives. Even if nobody goes berserk, depressed crew members can make mistakes or slack off when they should be exercising to prevent such hazards as cardiovascular deconditioning.

SexNASA officials worry that any kind of amorous relationship could be corrosive to crew cohesiveness. And if people do have sex, and should contraception fail, crew members might find their number increasing. Speculation along this line arose at a spring 2004 meeting of the British Interplanetary Society, where science fiction writer Rachel Armstrong accused NASA of discussing the need to have Mars-bound astronauts chemically sterilized. A spokesperson for the agency vehemently denied the scheme, so the baby-on-board issue remains.